Aerospace and Automotive Collaborate 2007 Supplement: Robotics Seeks Its Role in Aerospace
Articulated-arm robots, introduced to the automotive industry decades ago, are finding niches in aerospace
By Bruce Morey
When it comes to the technology of one industry finding acceptance in another, robotics moving from automotive to aerospace may be a classic case study, and one that is still in progress. Differences in the industries mean differences in the requirements that robots must meet. These go beyond the obvious ones, like the differences in product size and production volumes. Aerospace typically requires tighter tolerances, and uses lighter, stronger materials.
However, there are similarities. Automating precise, repetitive tasks that might tire—or injure—a worker is always a good thing. Low-cost flexibility is a quality the automotive industry values in places like body and paint shops. Aerospace may value them as highly if programming challenges, larger work envelopes, rigidity, and tighter tolerance and accuracy requirements can be met.
"Automation in the aerospace industry is a recent but not a new concept," according to Claude Perron, group leader automation robotics and intelligent manufacturing systems at the [Canadian] National Research Council Institute for Aerospace Research (Montreal, Quebec). He believes the need for high accuracy, repeatability, and quality in aerospace favors the use of automation. Aerospace has riveted and drilled automatically with gantry-style automation tools for years. However, gantry equipment, sometimes referred to as 'monuments,' is not always ideal. "A gantry-style machine is quite inflexible. If you change the aircraft model, you may have to buy a completely new gantry system. With a robot, all you have to do is change the programming," explains Perron.
Which is a nice idea that he is finding presents difficulties. Currently, his group is under contract to adapt robotics to a variety of aerospace applications, including metrology-assisted robotics. Their studies are mainly in high-accuracy processes, such as riveting and welding concepts like friction stir welding. Programming a robot for an aerospace application is a different challenge than in automotive, he contends. "In automotive, the tasks are normally simple in terms of trajectory, a few points being necessary to define the trajectory. In aerospace, you have to think in terms of thousands of different locations."
This requires the use of offline programming tools, bypassing the familiar teach pendant commonly used in programming robots. Fortunately, such offline programming tools are available, first developed in the automotive industry. His group uses Delmia V5 and tools supplied by robot manufacturers for programming robot workcells. Other tools he uses include Matlab/Simulink for simulation and Kuka VRC for real-time simulation required in collaborative tasks. He cites as desirable features of Delmia's robot simulation its compatibility with CATIA and a library of a large number of robot models from multiple vendors.
"The automotive industry is, however, ahead of the aerospace industry with regard to design for automation, or how to design a part such that its manufacturing could be easily automated," according to Perron.
As an early crossover application, painting and coating is ideal for making the leap from an automotive base to aerospace. "Everything we use for our aerospace applications, like all of the gun applicators and gun controls, were developed in automotive applications," explains Jerry Perez of Fanuc Robotics (Rochester Hills, MI).
He describes the requirements for aerospace as more demanding—like tighter tolerances over larger and more complicated surfaces. He describes one application where there is a need to spray precisely varying amounts of coatings over the nose cone of a particular aircraft, from thick at the bottom to thin at the top. This is a requirement Perez may never see in automotive, but the basic processes developed in that world are up to the challenge in the new one.
Perez recognizes the challenge of programming robot motions in the complex, large work envelopes encountered in aerospace. Fanuc has developed its own tool, PaintPRO, for robotic paint-cell process development. "The tool enables users that have a variety of part styles to save up to 50% of the time that it would take to program those parts manually [using a teach pendant]," says Perez. "We use this tool in all of our automotive applications, where we will produce over 80% of the programs before we even set foot in the paintshop." PaintPRO can accept CAD models that are in a variety of formats, including IGES and CATIA V5. Although a tool like PaintPRO simulates offline the teach-pendant approach, he admits that the next step is in developing automatic path generation, a needed tool for aerospace paint applications. "Fanuc Robotics has been developing this tool and has produced promising results for complex automotive and general industrial applications," says Perez. "We've been using it on Class 8 trucks and in the furniture industry."
While robotic coating has been common in aerospace for some time, it is less of a change from current automotive applications when compared to drilling applications, which require significant improvements in accuracy and rigidity, according to Todd Szallay, mantec development lead for Northrop Grumman's Integrated Systems sector. Northrop Grumman Corp. (El Segundo, CA) is conducting tests on articulated-arm robots for aerospace production-drilling applications.
Drilling is a high-volume operation in aerospace. Some aircraft require manufacturers to drill hundreds of thousands of precisely located, straight holes. "Recent improvements in payload capacity, accuracy, and rigidity are allowing robots to meet many aerospace requirements," Szallay says.
Drilling applications typically require a payload in the 240–360-kg range, according to Szallay. Robots in this payload range are now commonplace, off-the-shelf units. The higher payloads can both carry the drilling end-effectors and provide the rigidity needed for drilling holes in stacked layers of composite and titanium alloys. These improved rigidities prevent a skating effect on the surface of the material. The robot applies as much as 100 lb (45 kg) of force to the airframe.
His applications need accuracy as well as force. "Although some companies are offering off-the-shelf highaccuracy robots, they are relatively inaccurate when compared to large machine tools [such as gantry systems]," says Szallay. "A standard commercial off-the-shelf robot will give you positional accuracy of ±0.030" [0.76 mm] or so, which is good enough in some aerospace applications." If more accuracy is required, calibrated metrology systems (think various forms of machine vision) that can feed data to the robots can deliver even more accuracy.
"The tradeoff is the cost and complexity of the metrology or machine vision required to achieve the needed accuracy. The accuracy that's necessary is highly application dependent. What's important is that the more equipment you have to add to make the robot accurate, the less flexible and cost-effective it becomes. Robots can achieve single digits of accuracy, with these various add-ons." He sees a need for continued development to make robots even more accurate, without add-on equipment such as metrology. "What makes this technology so attractive is its cost, small footprint, and flexibility in use. Adding equipment means a more expensive robot that is less flexible." His group is currently testing and validating robotic-drilling concepts with stacked material coupons composed of typical aerospace materials, like epoxy composites and titanium.
Mike Beaupre, director emerging markets, Kuka Robotics (Clinton Township, MI) has seen aerospace robotics come a long way in seven years. "In 2000, we were developing a drilling project for Airbus in the UK, and there were very few robots in aerospace production," he says. "At that time, they were concerned that the robot was not accurate enough in positioning capability to meet their needs." Those concerns may not have been entirely satisfied in the intervening years, though he believes the robotics community is well on the way to full acceptance.
He believes articulated-arm robots achieving higher levels of accuracy are leading that acceptance. Achieving higher accuracy meant developing better calibration techniques, for both robots and workcells. He reports a 20% increase in accuracy through improved calibration, the procedure robots go through before they leave the factory. The company's high-accuracy calibration procedure uses 100 nodes, instead of the typical 35, to achieve the higher absolute accuracy.
They have a target accuracy in mind. "Although it's application-dependent, the target accuracy for applications such as drilling wing assemblies is ±0.015" [0.38 mm] absolute," explains Beaupre. "We are striving to get within that, and I think we're close." Achieving this goal would eliminate any need for extra equipment to achieve that accuracy target, making the robotic solution a standalone device. Once achieved, such accuracy could benefit the automotive world.
For instance, it might find a use in the ability of offline programming tools to deliver robot instructions even more conveniently, or facilitate mixing and matching robots without recalibrating their instruction set. Given the hypercompetitive nature of the automotive world, such accuracy might someday—soon—become the norm in automotive.
As important as accuracy is repeatability, the ability of a robot to repeat the same motion within a tolerance. Currently, robots in the 200–300 kg payload range are repeatable to ±0.2 mm, according to Beaupre, which he believes is sufficient for most aerospace applications.
Robotic automation may have a niche in the field of automation solutions. "I'm not sure we want to replace the large gantry-style machines, given the high level of accuracy they already achieve," says Beaupre. "Where we want to be is somewhere between all the manual labor that goes into building an aircraft and the large [gantry-style] monument machines. Our niche is in the middle."
Replacing manual labor may just be where robotics is most applicable to aerospace. Randy Schuetz, technology leader for Motoman Inc. (West Carrollton, OH), has developed a number of projects for aerospace customers, including painting, dispensing, and material-handling applications among others. Schuetz admits that it can sometimes be difficult to make the case for robotics to skeptical aerospace management and engineers. However, sometimes the strongest argument comes from a surprising quarter.
"We have had several projects where the production employees wanted the robots, saying that they were wearing themselves out, or that they had severe ergonomic issues," says Schuetz. "It's not like the production people are going to lose their jobs. They are going to be operating the robots rather than doing a manual operation."
Echoing a sentiment voiced by many others, he thinks the flexibility and reconfigurability of robots will eventually win the argument. "Because of the size of the product, we need to position the robot in different ways, combine them with gantries, or put them on rails to expand their workspace," he notes.
ABB Robotics (Auburn Hills, MI) has supplied a number of robots for aerospace applications, including deburring, sandblasting, drilling, riveting, grinding, polishing, and spot welding, according to Jim Winfree, chief engineer, aerospace. He reports that, for the applications they have delivered, their robots offer sufficient accuracy and repeatability, although he expects achieving both to be a continuing challenge. For example, for the drilling application the company is involved in, accuracy of about ±0.020" (0.51 mm) is required. ABB's absolute-accuracy optional advanced calibration process provides this accuracy.
One of the contrasts he notes between the industries is the lack of a supporting network of tooling suppliers in aerospace compared to automotive. "If you take, for example, a common automotive application like spot welding or arc welding, you can purchase off-the-shelf equipment from a variety of sources," says Winfree, noting that it's difficult to find commercial tools for robotic drilling of aerospace alloys or the layup of composite structures. Out of necessity, the aerospace industry develops much of its end-of-arm tooling, both in-house and with suppliers.
Machine vision and force control are developments to watch. "The use of machine vision within automotive is already commonplace, and can be transferred to aerospace," says Winfree. "Force control, on the other hand, is a more recent development—it's not in widespread use anywhere. We have some installations of force control in automotive. But I think the emergence of force control as a new technology, at a time of rising interest within aerospace, has a lot of potential."
He describes force control sensing as a special six-axis force-torque sensor on the end of the robot arm. The sensor provides feedback into the low-level servoloops that control the motors of the robot arm. "You use force feedback to maintain a constant force vector between the end of the robot arm and the workpiece," he explains. Typically, force control is combined with position control. Force control is used only when the end-effector is in contact with the workpiece—for example in polishing and grinding. Position control moves the end-effector to and away from the workpiece. Other force control technologies measure the current from direct-drive motors.
Robots will become more capable of replacing humans, even the human touch (through force feedback), though people will always be involved in aerospace manufacture. What human workers do may change drastically, however. Therefore, education of the workforce in robotics and the application of robotics will be a growing concern, according to Northrop Grumman's Szallay. "You need an educated workforce to go from a hand-held drill to operating a drilling robot. Technical training and certification programs are important. Aerospace will need people knowledgeable about both robots and the assembly process guiding these robots."
This article was first published in the October 2007 edition of Manufacturing Engineering magazine.